1. Field of the Invention
The present invention relates to sheet stacking devices in which a large number of sheets that are discharged thereto can be stacked and image forming apparatuses including such sheet stacking devices.
2. Description of the Related Art
With the advancement in technology, recently developed image forming apparatuses form images on sheets at an increasing speed. With such an increase in image forming speed, the speed of discharging sheets from the body of an image forming apparatus is also increasing. For the purpose of aligning and stacking a large number of sheets that are discharged at a high speed, there are some image forming apparatuses each including a large-capacity stacker device, a sheet stacking device, as disclosed in Japanese Patent Laid-Open No. 2006-124052.
FIG. 17 shows an exemplary known large-capacity stacker device. A stacker device 500 includes a gripper 503 that is attached to a conveying belt 508 rotating clockwise and moves along with the rotation of the conveying belt 508 while holding the leading end of a sheet, whereby the sheet is conveyed.
In the stacker device 500 having such a configuration, a sheet that is discharged from the body of an image forming apparatus (not shown) is first received by an entrance roller 501. Then, the leading end of the sheet is delivered by a conveying roller 502 to the gripper 503, and the conveying belt 508 rotates. In response to this, the gripper 503, which is holding the leading end of the sheet in combination with the conveying belt 508, moves along with the rotation of the conveying belt 508, whereby the sheet is conveyed above a sheet stacking table 505.
When the leading end of the sheet knocks against a leading end stopper 504, holding of the sheet by the gripper 503 is released, whereby the sheet falls and is stacked onto the sheet stacking table 505. Every time a sheet is stacked onto the sheet stacking table 505, an aligning unit (not shown) makes a jogging motion in a direction perpendicular to a sheet conveying direction (hereinafter denoted as the width direction) so as to align both sides of the sheet. Thus, alignment of stacked sheets is improved.
The stacker device 500 also includes a leading-end-pressing member 506 and a trailing-end-pressing member 507 that press the leading end and the trailing end, respectively, of a sheet stack SA on the sheet stacking table 505. While sheets are being stacked, the sheet stack SA is pressed by the leading-end-pressing member 506 and the trailing-end-pressing member 507 against the sheet stacking table 505 every time the number of sheets that have been stacked reaches a predetermined number. This facilitates discharging of subsequent sheets.
The stacker device 500 also includes a sheet surface detection sensor (not shown) configured to detect the position of the top surface of the sheet stack SA on the sheet stacking table 505. In accordance with a detection signal generated by the sheet surface detection sensor, the sheet stacking table 505 is lowered so that the top surface of the sheet stack SA on the sheet stacking table 505 is maintained at a level within a predetermined range. This enables continuous sheet discharge.
To remove the sheet stack SA on the sheet stacking table 505, an eject button is pressed, whereby the sheet stacking table 505 having the sheet stack SA thereon is lowered and is placed onto a dolly 509. After the sheet stacking table 505 is placed on the dolly 509, the dolly 509 is pulled out frontward in the depth direction in FIG. 17, whereby the sheet stack SA can be removed.
In the stacker device 500 having such a configuration, the sheet stack SA cannot be removed unless the sheet stacking operation of the stacker device 500 is stopped. Consequently, the image forming apparatus itself needs to be stopped to remove the sheet stack SA, leading to a reduction in productivity.
To avoid such a situation, an image forming apparatus connected to a plurality of sheet stacking devices is disclosed in Japanese Patent Laid-Open No. 2006-036533 (US Unexamined Patent Application Publication No. 2005/285334). In this image forming apparatus, when one of the sheet stacking devices becomes full of sheets, subsequent sheets are stacked in another sheet stacking device. Such a configuration enables a continuous sheet stacking operation. Thus, reduction in productivity can be prevented.
In the stacker device 500, the sheet stacking table 505 is raised or lowered by a motor controlled by a control unit, in such a manner as to be moved within a predetermined range. If the motor causes a malfunction because of electrical noise or the like, the sheet stacking table 505 may be moved beyond the predetermined range. To avoid such a situation, the known stacker device 500 includes a limiting mechanism that limits the sheet stacking table 505 not to be raised or lowered beyond the predetermined range. The limiting mechanism is provided on the body of the stacker device 500 and includes upper and lower stoppers. The sheet stacking table 505 is forcibly stopped when part of the sheet stacking table 505 knocks against the upper or lower stopper.
The known stacker device 500, however, has the following problem. The sheet stacking table 505 is lowered and is placed on the dolly 509 when a large number of sheets stacked thereon is removed. If the motor causes a malfunction in lowering the sheet stacking table 505, the sheet stacking table 505 that should be lowered may be accidentally raised. In the known stacker device 500, since the upper stopper limits the movement of the sheet stacking table 505 by having a direct contact therewith, the top of a sheet stack, if any, on the sheet stacking table 505 may bump into upper parts of the stacker device 500 before the sheet stacking table 505 is stopped by the upper stopper, leading to damage.
To avoid this problem, the sheet surface detection sensor provided to the stacker device 500 for detecting the position of the top surface of the sheet stack on the sheet stacking table 505 may be used so as to stop the upward movement of the sheet stacking table 505 due to malfunction in accordance with the detection by this sensor. Also in this case, however, the sheet surface detection sensor may likewise cause a malfunction because of electrical noise and become incapable of responding to a malfunction of the motor, resulting in incapability of stopping the upward movement of the sheet stacking table 505.